(1) Field of the Invention
The present invention relates to a radiation image pickup method and apparatus f o r acquiring a radiation image corresponding to the dose of radiation transmitted through an individual portion of an irradiation target. More particularly, this invention pertains to an arrangement which ensures control of the exposure for compensation for exposure.
(2) Description of the Related Art
A radiation image pickup apparatus comprises a radiation source, which is typically an X-ray tube, and an image pickup/recording device for detecting the radiation amount from the radiation source, which has been passed through an irradiation target, to pick up and record its corresponding radiation image. As the image pickup/recording device, a radiation sensitive film, a fluorescent display screen, and electronic image amplifying tube or the like may be used.
According to such a radiation image pickup apparatus, when a given dose of radiation is irradiated on individual portions of the irradiation target, proper exposure will result for some portions while underexposure will be caused for others due to the difference in the radiation absorbency of the individual portions. This phenomenon lowers the signal level corresponding to the underexposed portions in a device for photoelectrically reading a radiation projected image, or drops the density of the underexposed portion on an X-ray photograph. In either case, it is not possible to acquire sufficiently a desired amount of information for the under-exposed portion.
In photographing the chest portion of a human body with, for example, and X-ray examination apparatus used for medical purposes, therefore, it is not easy to reproduce both the image of such a portion as a lung, which easily passes radiation therethrough, and the image of such a portion as a backbone or abdomen, which does not allow easy transmission of the radiation, in a manner that permits easy simultaneous observation of these portions.
There is a conventional system which controls the exposure for each portion of the irradiation target in the following manner to acquire a proper exposure for that portion (see Japanese Unexamined Patent Publication No. Sho 62-129034).
The mechanism of controlling the exposure may be designed so that a fan-beam forming slit formed in a collimator provided between a radiation source and the irradiation target is provided with multiple shutter members arranged lengthwise of the slit and supported in such a manner as to be movable across the slit.
This mechanism scans a fan beam in a direction perpendicular to the lengthwise of the slit to irradiate radiation on the irradiation target, and detects the dose of radiation transmitted through the individual portions of the target. The mechanism then controls the amount of the diaphragm of the radiation by the shutter members (or the amount of the diaphragm of the slit area) based on the results of the detection to thereby permit image pickup while controlling the exposure to each portion of the target.
According to the above-described system which properly controls the exposure (the exposure to the irradiation target) by adjusting the opening area of the fan-beam forming slit, while a film or the like is exposed, the slit's opening area is immediately controlled on the basis of the dose of the exposure radiation, or the adjusting dose of radiation at the actual image-pickup is acquired from preacquired information of the dose of radiation transmitted through the irradiation target, so that the image pickup is carried out separately by controlling the radiation dose based on the control dose.
In either case, according to the prior art, an image is picked up by a single irradiation executed while controlling the exposure to the irradiation target. To secure the maximum value of radiation dose necessary for the irradiation target (the radiation dose necessary for the proper exposure at a portion which least passes radiation through), therefore, the output of the radiation source is kept large for the image pickup. For an irradiation target whose average level of the necessary dose of radiation is small, image pickup is executed with a large dose of radiation cut down. This method is likely to waste much portion of the radiation dose from the radiation source and naturally puts a large burden on radiation generated from the radiation source.
When there is some portion for which compensation for exposure variation, if any is not desirable, or when there is an image for which no compensation for exposure should be made at all, the prior art cannot automatically cope with either case, and requires that, for example, an operator (photographer) determine in advance a region for the compensation for exposure, or operate a switch to cancel the compensation. The required operation is troublesome and increases a burden on the operator to determine such a region.
Particularly, in an X-ray examination apparatus available for medical purposes, if the difference in exposure caused by some disease is compensated for, medical diagnoses of the disease may be adversely affected. In this respect, there is a demand for a system that can control the radiation dose in different ways, but the conventional apparatus has had difficulty in fulfilling such demand.